An apparatus of this type can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer), which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies, which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatus—which is commonly referred to as a step-and-scan apparatus—each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since the projection system will have a magnification factor M (generally <1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine
In currently available lithographic devices, the employed radiation is generally ultra-violet (UV) light, which can be derived from an excimer laser or mercury lamp, for example; many such devices use UV light having a wavelength of 365 nm or 248 nm. However, the rapidly developing electronics industry continually demands lithographic devices which can achieve ever-higher resolutions, and this is forcing the industry toward even shorter-wavelength radiation, particularly UV light with a wavelength of 193 nm or 157 nm. Beyond this point there are several possible scenarios, including the use of in-band extreme UV light (EUV:wavelength˜50 nm and less, e.g. 13.4 nm, 13.5nm or 11 nm), X-rays, ion beams or electron beams. All of these so-called next-generation radiations undergo absorption in air, so that it becomes necessary to at least partially evacuate the environment in which they are employed. This introduces considerable problems.
A general discussion of the use of EUV in lithographic projection apparatus can be found, for example, in the article by J. B. Murphy et al. in Applied Optics 32 (24), pp 6920-6929 (1993). Similar discussions with regard to electron-beam lithography can be found in U.S. Pat. No. 5,079,112 and U.S. Pat. No. 5,260,151, as well as in EP-A 98201997.8 (P-0113.000-EP).
The European patent application EP 0 957 402 A2, which is incorporated in the present application by reference, describes a lithographic apparatus, whereby a projection system is separated from a substrate table by an intervening space. The intervening space can at least partially be evacuated. The intervening space contains a hollow tube, which is continually flushed by a flow of gas, to reduce cross-contamination between the substrate table and the projection system. The gas does not substantially absorb EUV radiation. According to EP 0 957 402 A2, the gas is Ar or Kr. During use, radiation is directed through the tube, from the projection system to a substrate which is held by the substrate table. An advantage of the use of argon is that the ‘removability’ or ‘pumpability’ of argon is relatively good. For instance, in typical vacuum pumps used in lithographic apparatus, such as turbomolecular pumps, the pumping speed for argon (relative molecular mass 40) is relatively large compared to the pumping speed for very light gases, such as hydrogen (relative molecular mass 2), or very heavy gases like Xenon (relative molecular mass 131).
International application WO 01/84241 A1, which is also incorporated herein by reference, describes a lithographic apparatus and method, comprising a non-contact seal using a purge gas. Therein, a purged optical path between an optical source surface and an optical target surface is provided, as well as relative movement between the optical source surface and the optical target surface. A purge gas of a controlled purity is used.